In GERAN (GSM/EDGE Radio Area Network) Iu mode at present, the MAC (medium access control layer is responsible for the mapping between the logical channels (traffic or control channels) and the basic physical subchannels (Dedicated Basic Physical SubCHannel or Shared Basic Physical SubCHannel). The logical channels are the channels the physical layer offers to the MAC layer. These logical channels and the mapping to the basic physical subchannels are fully specified in GSM/EDGE standards, allowing the functionality in the MAC layer to be relatively simple.
A different approach is taken in UTRAN (UTMS Terrestrial Radio Access Network) where, instead of providing logical channels, the physical layer offers Transport Channels (TrCH), which can be used by the MAC layer. A transport channel can be used to transmit one flow over the air interface. A number of transport channels can be active at the same time and are multiplexed at the physical layer. The transport channels are configured at call set-up by the network.
The concept of transport channels is proposed to be used in GERAN. Each of these transport channels can carry one flow having a certain Quality of Service (QoS). A number of transport channels can be multiplexed and sent on the same dedicated physical subchannel thereby making it possible to have different protection on different classes of bits, for instance. The configuration used on a transport channel i.e. the number of bits, coding, interleaving etc. is denoted the Transport format combination (TF). As in UTRAN, a number of transport format combinations can be associated with one transport channel. For instance, in adaptive multirate encoding (AMR), the class 1a bits have their own TrCH, with one transport format combination configured per AMR mode. The configuration of the transport format combinations can be controlled by the network and signalled to the mobile on call set-up. In both the mobile and the BTS, the transport format combinations can be used to configure the encoder and decoder units. When configuring a transport format combination, the network can choose between a number of predefined CRC (cyclic redundancy check) lengths and code types. For each of the transport channels, a given number of transport format combinations can be configured on call set-up.
Transport blocks (TB) are proposed to be exchanged between the MAC layer and the physical layer on a transport time interval (TTI) basis (e.g. 20 ms). For each transport block a transport format combination is chosen and indicated through the transport format combination indicator (TFI). In other words, the TFI tells which channel coding to use for that particular transport block on that particular TrCH during the TTI.
Only some combinations of the transport format combinations of the different TrCH are allowed. A valid combination is called a Transport Format Combination (TFC). When transport format combinations are combined in a TFC the sum of the output bits adds up to the total number of available bits in a radio packet on the basic physical sub-channel e.g. 464 bits for Gaussian minimum shift keying (GMSK) full-rate channels. The set of valid TFCs on a physical sub-channel is called the Transport Format Combination Set (TFCS).
In order to decode a received sequence, the receiver needs to know the active TFC for a radio packet. This information is transmitted in the Transport Format Combination Indicator (TFCI) field. This field is a layer 1 header, and has the same function as the stealing bits commonly used at present. Each of the TFC within a TFCS is assigned a unique TFCI value, which is the first thing to be decoded by the receiver when a radio packet is received. From the decoded TFCI value, the transport format combinations for the different transport channels can be found, allowing decoding to start.
FIG. 1A shows a proposed architecture for a GERAN flexible layer one. Although it is inspired by the architecture that was standardised for the UL in UTRAN, it is significantly more simple.
Referring to FIG. 1A, a physical layer includes the following processes in sequence in respect of each TrCH provided by a layer two above: CRC attachment, channel coding, radio segment equalisation, first interleaving, segmentation, rate matching, transport channel multiplexing, TFCI mapping and second interleaving. In the CRC attachment step, error detection is provided on each transport block through a CRC. The size of the CRC to be used is fixed on each TrCH and is configured by a radio resource layer (RRC) higher than the layer one, and is a semi-static attribute of the transport format combination. The entire transport block is used to calculate the parity bits. Code blocks are output from the CRC attachment process.
Code blocks are then processed by the channel coding process, producing encoded blocks. The channel coding to be used is chosen by the RRC and can only be changed through higher layer signalling. The channel coding used is a semi-static attribute of the transport format combination, although in practise it will probably be fixed for each TrCH. Thus, for AMR, the same channel coding is used for all the modes, and rate matching simply adjusts the code rate by puncturing or repetition. In the radio segment equalisation step, radio segment size equalisation adjusts (by padding) the input bit sequence to ensure that the encoded block can be segmented into Si data segments of same size. The first interleaver is a simple block interleaver with inter-column permutation. Its task is to ensure that no consecutive coded bits are transmitted in the same radio packet.
When the TTI is longer than the radio packet duration, the input bit sequence is segmented by the segmentation process, and each Si radio segment is mapped onto one radio packet (Si=Transmission time/radio packet duration). As a result, the input bit sequence is mapped onto Si consecutive radio packets.
The three last described processes (equalisation, first interleaving and segmentation) are only used when the TTI is longer than the radio packet duration, and are transparent otherwise. For each encoded block, they produce Si radio segments.
The rate matching process is the core of the flexible layer one. It causes bits of a radio segment on a transport channel to be repeated or punctured. Layers above the layer one assign a rate matching attribute for each transport channel. This attribute is semi-static and can only be changed through higher layer signalling. Once the number of bits to be repeated or removed is calculated, rate-matching attribute can begin. The higher the value of the attribute, the more important the bits (more repetition/less puncturing). Since the block size is a dynamic attribute, the number of bits on a transport channel can vary between different transmission times. When this happens, bits are repeated or punctured to ensure that the total bit rate after TrCH multiplexing is identical to the total channel bit rate of the allocated dedicated physical channels. Data output from the rate matching process is termed a radio frame. For every radio packet to be transmitted, the rate matching produces one radio frame per radio segment, e.g. per TrCH.
In the TrCH multiplexing step, one radio frame from each TrCH is delivered to the TrCH multiplexing, for every radio packet to be transmitted, according to the TFC. These radio frames are serially multiplexed into a coded composite transport channel (CCTrCH). For every radio packet to be transmitted, the coded TFCI is attached at the beginning of the CCTrCH by the TFCI mapping process before interleaving. The coded TFCI and the CCTrCH are interleaved together by the second interleaving step on radio blocks. The interleaving can be either diagonal or block rectangular, and is configured on call set-up.
An alternative architecture is shown in FIG. 1B. Here, the radio segment equalisation, first interleaving and segmentation processes of the FIG. 1A architecture are omitted.